Base reach-through active series voltage regulator

ABSTRACT

The pass transistor of this series regulator supplies current at a regulated voltage only sightly below its unregulated supply voltage. The driver transistor, driven by the feedback loop, is supplied from a higher voltage source. During periods when the pass transistor cannot supply the load because its source voltage is too low, the driver transistor supplies the load through the base-emitter junction of the pass transistor without affecting the regulation.

BACKGROUND OF THE INVENTION

This invention relates to regulated DC power supplies. Moreparticularly, it relates to what is known as the series type regulatorin which regulation is achieved by the control of an impedance elementbetween input and output. This is distinguished from the switching typeregulator in which the duty cycle of a switch connected between inputand output is controlled to effect regulation.

The choice between a series type or switching type DC voltage regulatoris often a difficult one, particularly for an audio frequencycommunications system. The switching type regulator is very efficient,but it generates considerable noise within or slightly above the audiofrequency range that can require filtering at an appreciable expense.The series type regulator, on the other hand, is in itself a dynamicfilter that provides excellent regulation, but it is notoriouslyinefficient. The entire expected range of variation of input voltagemust usually be absorbed within the series element. While in alaboratory power supply efficiency is not usually a very importantconsideration, in a large system, it can be very important. In atelephone system, for example, with thousands of circuits to be suppliedwith regulated voltage, efficiency must be considered with respect toheat and space limitations, as well as the cost of power consumption.

In U.S. Pat. No. 3,414,802, which issued to this inventor and W. E.Jewett on Dec. 3, 1968, a system is disclosed that solves the efficiencyproblem with a pair of stacked series regulators. The Harrigan-Jewettsystem uses two regulators with their outputs connected in parallel. Thefirst regulator operates efficiently with a minimum voltage drop acrossit during the very high percentage of time when the input voltage iswithin a narrow nominal range. The second regulator, operating from ahigher input voltage, is adjusted to supply a slightly lower outputvoltage than the first regulator supplies. The second regulator istherefore normally cut off and operates only when the output voltagefalls because of a larger drop in the input voltage than the firstregulator can absorb. The system is therefore particularly useful wheremore than one supply voltage is available and where the normal inputvoltage variation is not great. The system does not require however, tworegulators each complete with output voltage sensor, error detectorfeedback circuit, and pass transistor.

An object of this invention is to regulate DC voltage very efficientlyover a wide input voltage range using a single series type regulator.

A second object is to provide efficient DC voltage regulation atconsiderable savings in cost and space.

A third object is to provide the same advantages of the Harrigan-Jewettcircuit with the use of a single series type voltage regulator.

SUMMARY OF THE INVENTION

In this efficient series type voltage regulator, the collector of thepass transistor is connected to a first source of direct current, andthe emitter is connected to the regulated voltage output terminal. Thecollector-emitter path of a driver transistor is connected between asecond DC source and the base of the pass transistor. Feedback means areconnected between the output terminal and the driver transistor to drivethe driver transistor in response to the output voltage. The differencebetween the voltage of the first source and the regulated output voltageis so small that the first source voltage occasionally drops below theregulated voltage. During such periods, the driven transistor suppliesthe current to the output terminal through the base-emitter junction ofthe pass transistor from the second source, the voltage of which is highenough to remain above the regulated voltage. Efficiency is improvedbecause the voltage drop across the pass transistor is never very large,and the current through the driver transistor is large only duringperiods of very low first source voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic diagram of a circuit embodying the essentialelements of the invention;

FIG. 2 is a schematic diagram of a particularly useful embodiment of theinvention, and

FIG. 3 is a schematic diagram of an alternative embodiment of theinvention.

DETAILED DESCRIPTION

In the embodiment of FIG. 1, a series voltage regulator 11 is powered bytwo direct current supplies 14 and 16. Supply 14 is connected through ablocking diode 15 to the collector of the pass transistor 21 ofregulator 11. The emitter of pass transistor 21 is connected to anoutput terminal 20 to which a load 22 may be connected. Thecollector-emitter path of a driver transistor 23 is connected betweensupply 16 and the base of pass transistor 21. A potentiometer 31 isconnected between output terminal 20 and ground. The series combinationof a zener diode 32 and a resistor 33 is also connected between outputterminal 20 and ground. Finally, an error detector transistor 34 has itscollector connected to the base of driver transistor 23, its emitterconnected to the junction between zener diode 32 and resistor 33 and itsbase connected to the tap on potentiometer 31.

Except for the fact that driver transistor 23 and pass transistor 21operate from different current supplies, the circuit of FIG. 1 operatesin a manner similar to the well-known series type regulator. The zenerdiode 32, being biased into its breakdown range by resistor 33,maintains a relatively constant voltage across its terminals. Ittherefore applies directly to the emitter of error detector transistor34 the entire variation of output voltage. The base of transistor 34 onthe other hand, being connected to a potentiometer tap, sees only aportion of the variation of output voltage. The potentiometer tap actslike a source of reference potential. If the output voltage at terminal20 tends to rise therefore, the emitter voltage of transistor 34increases much more than its base voltage, reducing its collectorcurrent. Applied to the base of driver transistor 23, the reducedcollector current reduces the drive to base transistor 21, in turnreducing the current to terminal 20, hence offsetting the originaltendency of the output voltage to rise. This provides the well-knownseries type regulation which is efficient only when the drop across thepass transistor is small.

In order to keep the voltage drop across pass transistor 21 small forhigh efficiency, regulator 11 is designed as if it were handling only anominal range of supply voltage from supply 14. That is, the regulatedoutput voltage at terminal 20 plus the minimum drop across transistor 21is equal to the nominal minimum output voltage of supply 14. The nominalminimum voltage may be considered the minimum voltage that occursrelatively frequently during regular power service. With the exceptionof the regulator of the Harrigan-Jewett patent noted before, voltageregulators designed in this manner cannot handle the larger inputvoltage variations that occur relatively infrequently. Sustainedoperation as standby battery sources during commercial power blackout,for example, would result in a drop in output voltage that defeats thepurpose of the regulator. According to the principles of this invention,however, driver transistor 23 is supplied from supply 16, the voltage ofwhich remains higher than that required at output terminal 20. When theinput voltage to pass transistor 21 drops far enough that itscollector-emitter junction is voltage limited and therefore cannotsupply all of the current called for to maintain the output voltage atterminal 20, driver transistor 23, operating from a higher voltagesource, is not so voltage limited. It then begins to supply not merelydrive current, but output current as well, right through thebase-emitter junction of transistor 21 to maintain the output voltage.In order to provide this "reach-through" regulation, of course,transistor 23 and the base-emitter junction of transistor 21 must eachbe large enough to handle the whole load current. In addition, thenominal voltage of supply 16 must be high enough that it exceeds theoutput voltage at all times, even during periods when the voltage ofsupply 14 does not. Diode 15 prevents reverse collector current intransistor 21 driven by transistor 23.

If the basic source voltage of supply 16 varies considerably, arelatively large voltage drop will be required across transistor 23 mostof the time in order to insure adequate minimum voltage. This is theconsideration in connection with the pass transistor that generallycauses series regulators to be very inefficient. In this case, however,during the large percentage of time transistor 23 is supplying only basecurrent to transistor 21 and hence is not wasting much power. It is onlyduring periods of unusually low voltage at supply 14 that transistor 23supplies the load. If the voltage of supply 16 is also low during thesesame periods, the power waste is low even then.

During normal operation, therefore, load current is supplied from supply14 through transistor 21 efficiently because the voltage acrosstransistor 21 is never very large. During periods of abnormally lowvoltage at supply 14, the collector of transistor 21 is backbiased, andall of the load is supplied from supply 16 through transistor 23 and thebase-emitter junction of transistor 21 with no drop in output voltage.

FIG. 2 shows in schematic form an embodiment of the invention that isparticularly useful in supplying direct current of regulated voltagefrom a power system in which standby batteries are floated across thecommercial power. In this figure, components and circuits have beengiven the same numbers as their counterparts in FIG. 1 for clarity. Thetwo supplies in this case are both derived from one inverter 110 whichhas two input leads 112, and 113 for connection to the DC power source.The particular inverter shown in an improved two-core Jensen typeinverter that is the subject of U.S. Pat. No. 3,448,370, which issued tothe inventor of this application, June 3, 1969. Any reliable invertermay, of course, be used in its place. The secondary winding 117 of theoutput transformer of inverter 110 has a center tap 118, and twosymmetrical taps 119--119 to provide the two supplies of FIG. 2. A setof diodes 115--115 connected to the ends of winding 117 and to each tapprovide full-wave rectification for each supply, and filter capacitors141--141 are connected across each supply to smooth the input voltagesto the regulator. The outputs of these supplies therefore appear onconductors 114 and 116, respectively. Pass transistor 21 is connectedbetween conductor 114 and output terminal 20, and driver transistor 23is connected between conductor 116 and the base of transistor 21, as inFIG. 1.

The standby batteries floating across input leads 112 and 113 act aslarge filters and hence provide some voltage regulation themselves. Thevoltage variation on output 114 of inverter 110, therefore, is not verylarge as long as the commercial input power is uninterrupted.Transformer winding 117 is designed so that the minimum nominal voltageon output lead 114 with commercial power supplied is about 1 volt higherthan the required regulator output voltage at terminal 20. There areenough turns in remainder of the secondary winding, however, so thateven after the commercial power has been off for several hours, thevoltage on lead 116 is still a few volts higher than the required outputvoltage. To provide enough gain in the feedback loop, an additionaltransistor 142 has been connected to drive transistor 23 in thewell-known Darlington amplifier fashion.

The required output voltage is supplied very efficiently from this lowcost equipment even during commercial power failure. If standby power isneeded longer than the several hours available from the batteries,auxiliary generating equipment can be started and connected to rechargethe batteries with no interruption in the regulated output voltage.

An alternative regulator embodying the principles of the invention andusing only NPN-transistors is shown in FIG. 3. There are three areas inwhich this regulator differs from that of FIG. 2, all arising from theuse of transistors of a single polarity. First, the feedback amplifiertransistor 242 is connected in a common emitter configuration ratherthan a Darlington configuration. In order to provide drive to transistor23, transistor 242 needs somewhat more reserve voltage because of itspolarity. It is accordingly connected to a still higher voltage source243 derived from additional turns 244--244 on transformer winding 217.With amplifier 242 operation from a higher voltage source however, whentransistor 23 becomes voltage limited so that it cannot supply theoutput current through the base-emitter junction of transistor 21,transistor 242 would attempt to do so by reaching through thebase-emitter junction of transistor 23 in the same manner. Transistor246 and resistor 247 are therefore connected as a current limiterbetween transistors 242 and 23. As the emitter current of transistor 242increases to increase the voltage drop across resistor 247, transistor246 becomes more conductive to shunt the drive to transistor 242 andtherefore limit the current. Finally, zener diode 232 is connectedbetween the negative output terminal and the emitter of error detectortransistor 34. The output voltage variations sensed by the tap onpotentiometer 31 are applied to the base of transistor 34 to develop theerror signal.

It will be obvious to those skilled in the art of voltage regulatorsthat many variations can be made from the circuits specificallydescribed including, for example, reversing the polarities of all of thetransistors without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A direct current voltage regulator for providing.Iadd.a load current at .Iaddend.a regulated voltage to an outputterminal comprising a first source of direct current having a voltagethat may drop below said regulated voltage, a pass transistor having itscollector connected to said first source and its emitter connected tosaid output terminal, a second source of direct current having a voltagethat remains above said regulated voltage, a driver transistor havingits .[.collector-emitter path.]. .Iadd.collector and emitter.Iaddend.connected .Iadd.in a low impedance path .Iaddend.between saidsecond source and the base of said pass transistor, and feedback meansconnected between said output terminal and said driver transistor fordriving said driver transistor in response to the voltage at said outputterminal .[.whereby during periods when said first source voltage isbelow said regulated voltage.]., said driver transistor .[.supplies.]..Iadd.operating to supply said load .Iaddend.current .Iadd.at saidregulated voltage .Iaddend.to said output terminal through thebase-emitter junction of said pass transistor .[.to maintain.]..Iadd.during periods when said first source voltage is lower than.Iaddend.said regulated voltage.
 2. A direct current voltage regulatoras in claim 1 wherein said first and second direct current sources arederived from a common alternating current source.
 3. A direct currentvoltage regulator as in claim 1 wherein the polarity of said drivertransistor is opposite to the polarity of said pass transistor.
 4. Adirect current voltage regulator as in claim 3 wherein said feedbackmeans includes a source of reference potential, means for sensing thevoltage at said output terminal, comparing means for producing an errorsignal proportional to the difference between said regulated voltage andsaid reference potential and amplifying means for driving said drivertransistor in proportion to said error signal.
 5. A direct currentvoltage regulator as in claim 2 including a third DC source derived fromsaid common AC source connected to said feedback means for providing theenergy to drive said driver transistor, and current limiting meansconnected between said feedback means and said driver transistor wherebysaid feedback means are prevented from supplying current to said outputterminal through the base-emitter junction of said driver transistor.